The present invention relates to an isolation method of semiconductor devices which can minimize an isolating region by preventing the formation of bird's beak.
Recently, as the semiconductor devices have a tendency toward high integration, the isolation region which is to electrically isolate adjacent active regions should be reduced in proportion to the reduction of the chip size. Particularly, the size of the isolation region is a major factor for determining the size of a memory device and thus lots of investigation has been developed to reduce the size of the isolating region.
Generally, a local oxidation of silicon method (LOCOS), trench, field shielding, and so on have been employed for such an isolation method, where the LOCOS method is widely used in such a manner as to form the isolation region by oxidation at high temperature after formation of oxide and nitride layers on the surface of a silicon substrate and subsequential removal of the nitride layer from an isolation region.
However, there is a problem in isolation of narrow active regions due to the bird's beak which extends from the isolation region into the active region during the high temperature oxidation. Furthermore, the stress during oxidation can result in dislocation that causes a subsequent leakage current at P-N junction.
Therefore, the LOCOS method is not suitable for the isolation of submicron devices. As a consequence, the trench and field shielding isolation methods have been proposed for such submicron devices. The trench isolation method can achieve a perfect isolation of the sub-micron devices. However, there are such problems as deterioration of junction and transistor characteristics. Special equipments and techniques are required to suppress the crystalization defect resulted from the substrate etching during trench formation.
On the other hand, field shielding isolation has a trouble in interconnection since bias should be applied to a field plate. Moreover, the current leakage between the field plate and other electrodes should be eliminated appropriately.
As apparent from the description heretofore, it is very difficult to form the isolation effectively. Various methods proposed to prevent abovementioned problems are not applicable to practical products because of specific techniques and apparatus.